The switchgear can be used to control an electric motor connected downstream from the switchgear. To this end, energy is supplied to the electric motor connected downstream from the switchgear via the switchgear so that the electric motor is connected to a supply network. The supply network is in particular a low-voltage network for an industrial installation (for example three phases, 400 volt, 50 hertz).
To control the energy supply of the downstream electric motor, the switchgear comprises a control unit and a first current path. A first phase of the supply network is supplied to the downstream electric motor via the first current path. The first current path comprises a first electromechanical switch and, connected in series to the first switch, a parallel connection of a second electromechanical switch with a semiconductor switch. The control unit can output a switching signal for the first switch, the second switch and the semiconductor switch and in this way, control the desired switching state of the switches. The first and second switches are in particular make-contacts, which, in the presence of a switching signal of the control unit, are held in the closed switching state. If the switching signal is switched off by the first or second switch, due to a restoring force (for example a spring force acting on a contact of the switch), the switch automatically adopts the open state. Switches of this kind are in particular relays. In the presence of the switching signal of the control unit, the semiconductor switch preferably adopts the electrically conducting state.
The switchgear comprises a supply connection via which the control unit can draw the energy for the switching signals. Usually, a decentralized power supply source is connected via a line to the supply connection of the switchgear so that, in the active operation of the switchgear, a supply voltage is applied through the supply connection. The switchgear draws the energy required for the switching signals via the supply connection. Preferably, the entire internal energy supply of the switchgear is drawn through the supply connection of the switchgear.
The switchgear is preferably used to switch three-phase motors and also single-phase AC motors on and off and to protect the motors against overload. Preferably, the switchgear can also be used in safety applications. In this case, it is necessary to ensure that the motor is reliably switched off.
In the case of switchgears with emergency shutdown of the motor connected to the switchgear, one possible shutdown principle is to switch off the supply voltage of the switchgear by use of a switchgear inserted into the supply line between the power supply source and the supply connection (for example an emergency-stop switchgear). Functionally, the removal of the supply voltage causes the electromechanical switches of the switchgear automatically to adopt the off condition (i.e. the switches are opened). In this way, when the supply voltage is removed, the energy supplied via the switchgear to the motor is switched off so that the motor is reliably switched off. Switching-off the power supply source and the automatic opening of the switches causes a circuit-breaking arc to form at the switches, which causes heavy wear at the switches so that the number of switching operations of this kind is usually limited for the switchgears.